Uma, At the London meeting we presented a sample of results from modeling different PQ-selection heuristics in real network topologies. They are in slides 7-14 of this link.
http://www.ietf.org/proceedings/89/slides/slides-89-rtgwg-4.pdf The comparison of per-PLR coverage for different heuristics for selecting a subset of PQ-nodes is shown for two topologies on p.11-14. Not shown in these slides are the results when one considers all possible PQ-nodes, but this was also computed. The exhaustive PQ-node results show that for some topologies, there is potential to improve coverage with more PQ-nodes or a better choice of PQ-nodes, especially with respect to node-protecting coverage. However, for these two topologies, even with a simple heuristic for selecting 16 PQ nodes, RLFA offers a significant improvement in coverage compared to local LFA (as shown on p.9-10 ), so some service providers may find it useful. As has been pointed out by others, having to deal with the constraint imposed by limiting possible repair tunnels to those that can be created with a single LDP label (extended P-space) is generally going to be problematic. Using explicitly routed repair tunnels (created using RSVP or segment routing) or using an alternate topology (as with MRT) may be more productive approaches to improving coverage and backup path quality. Chris From: Uma Chunduri [mailto:[email protected]] Sent: Tuesday, March 18, 2014 1:19 AM To: Pushpasis Sarkar; [email protected]; Alia Atlas Cc: [email protected]; [email protected]; Chris Bowers Subject: RE: WG Adoption Call for draft-psarkar-rtgwg-rlfa-node-protection In-line [Uma1]: -- Uma C. From: Pushpasis Sarkar [mailto:[email protected]] Sent: Monday, March 17, 2014 11:14 AM To: Uma Chunduri; [email protected]<mailto:[email protected]>; Alia Atlas Cc: [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; Chris Bowers Subject: RE: WG Adoption Call for draft-psarkar-rtgwg-rlfa-node-protection [Uma]: So we do few hundreds of fSPF from each neighbor for base LFA then few hundreds for Q space rSPFs and few more hundreds for rSPFs from PQ nodes depending on the network/heuristics for each primary SPF right. Did you see your 1msec is multiplied few multiple hundred times here. [Pushpasis] Few hundreds fSPFS for PQnodes are not necessary. 8-16 PQ nodes selected using the default heuristic in this draft provide quite a good amount of coverage. [Uma1]: "8-16 PQ nodes/ quite a good amount of coverage/ few additional fSPFs (~10)" Well, we all know the above qualifications are purely subjective and completely depends on the topology, number of total nodes, number of total interfaces etc.. If the total number of candidate PQ nodes are in the order of multiple hundreds and with LFA manageability considerations like per prefix options are enabled limiting to a marginal value to 8-16 PQ nodes may not give the gains we are seeking to start with. I think we are back to heuristics. However, I know with some tricky heuristics we can limit the fSPFs and total number of candidate PQs.
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